U.S. Pat. No. 4,840,460 discloses a liquid crystal display (LCD) panel typical of the prior art display panel of the type discussed herein. In this patent, as shown in FIGS. 1A and 1B illustrating plan and cross-sectional views, respectively, each of the pixel electrodes 13 facing a common electrode 12 with a liquid crystal layer 11 interposed therebetween is divided into a plurality of (four in the illustration) subpixel electrodes 13.sub.1 -13.sub.4 having equal surface areas. Disposed in facing relation with the divided subpixel electrodes 13.sub.1 -13.sub.4 are corresponding control capacitor electrodes 15.sub.1 -15.sub.4 having different surface areas, the control capacitor electrodes being separated from the subpixel electrodes by an insulation layer or dielectric layer 14. All the control capacitor electrodes 15.sub.1 -15.sub.4 in each display pixel are electrically connected with each other. More specifically, the common electrode 12 is disposed on the inner surface of a transparent substrate 16 while the control capacitor electrodes 15.sub.1 -15.sub.4 are formed on the inner surface of a second transparent substrate 17 disposed in spaced opposing relation with the first transparent substrate 16. Disposed on those control capacitor electrodes 15.sub.1 -15.sub.4 is the insulation layer 14 on which the subpixel electrodes 13.sub.1 -13.sub.4 are formed.
The pixel electrodes 13 are arrayed in the form of a matrix on the inner side of the transparent substrate 17. A source bus 18 is provided along each column of the array of pixel electrodes 13 on the inner side of the transparent substrate 17 while a gate bus 19 iS provided along each row of the array of pixel electrodes 13 on the insulation layer 14.
Formed in the vicinity of the intersection between each source bus 18 and the associated gate bus 19 is a thin film transistor 21 which comprises a source electrode 22 and a drain electrode 23 both formed on the inner side of the transparent substrate 17, a semiconductor layer 24 of a material such as amorphous silicon formed between the source and drain electrodes 22, 23, and a gate electrode 26 disposed on the semiconductor layer 24 with a gate insulating film 25 interposed therebetween. The gate insulating film 25 may be formed concurrently with formation of the insulation layer 14. The source electrode 22, the drain electrode 23 and the gate electrode 26 are connected with the source bus 18, the control capacitor electrodes 15.sub.1, and the gate bus 19, respectively.
The control capacitor electrodes 15.sub.1 -15.sub.4 and the the corresponding opposing subpixel electrodes 13.sub.1 -13.sub.4 separated therefrom by the insulation layer 14 compose control capacitors 27.sub.1 -27.sub.4, respectively. The subpixel electrodes 13.sub.1 -13.sub.4 and the opposing common electrode 12 separated therefrom by the liquid crystal layer 11 compose liquid crystal capacitors 28.sub.1 -28.sub.4, respectively.
These two types of capacitors are interconnected such that each one of the capacitors of one type is equivalently connected in series with the corresponding one of the capacitors of the other type through an external drive circuit (not shown), as illustrated in FIG. 1C. The driving voltage supplied from the source buses 18 to the control capacitor electrodes 15.sub.1 -15.sub.4 through the thin film transistors 21 is thus divided by the two types of capacitors.
While the capacitances of the LC capacitors 28.sub.1 -28.sub.4 are equal, the capacitances of the control capacitors 27.sub.1 -27.sub.4 are different from each other, so that even if the driving voltage is applied equally to the control capacitor electrodes 15.sub.1 -15.sub.4 from the common source, the capacitance-divided voltages applied to the associated LC capacitors 28.sub.1 -28.sub.4 may be different from one another. Since the threshold voltage of the liquid crystal 11 is substantially constant over the entire surface of the LCD panel, it is possible to control the numbers of the subpixel electrodes in which the respective capacitance-divided voltages applied to the liquid crystal 11 are higher than the threshold voltage and the sub pixel electrodes in which the respective capacitance-divided voltages applied to the liquid crystal 11 are lower than the threshold voltage, by controlling the applied voltage to the control capacitor electrodes 15.sub.1 -15.sub.4, and hence it is possible to drive the divided domains of the display pixel in a stepwise manner.
When an LCD panel is used to display images containing halftones such as those of a television display, the driving voltages supplied to the respective pixel electrodes of the LCD panel may have various magnitudes within a certain range of voltages according to the image signal levels. In an LCD panel in which no pixel electrode is divided into subpixel electrodes, the gray scale display takes place utilizing the inclined region of the transmittance curve of the display pixel region varying from a start-up to a saturation with an increase in the driving voltage. In the inclined region of this transmittance curve the liquid crystal molecules are oriented diagonally with respect to the substrate. Since the transmittance in this state depends greatly on the viewing angle, the appropriate viewing angle for such a LCD panel is usually considerably narrow.
In contrast, in such a pixel as disclosed in the aforesaid U.S. patent where each of the pixel electrodes is divided into a number of subpixel electrodes which are adapted to be supplied with successively varying applied voltages, the subpixel electrode sections will successively reach the saturation region through the inclined region of the transmittance curve as the driving voltage increases such that the transmission of one of the subpixel electrode sections will rise up and reach the saturation region through the inclined region of the transmittance curve, then another one will do the same, and so on.
Consequently, in a state in which any half tone is on display, the LC molecules are oriented diagonally with respect to the substrate in at most one of the subpixel electrode sections, but are oriented either substantially vertically or substantially horizontally in the remaining subpixel electrode sections. By minimizing the areas where the LC molecules are diagonally oriented in the halftone displaying condition as described above, it is possible to decrease the area of the display pixel region in which there is a high viewing angle dependence and hence reduce the average viewing angle dependence of the entire pixel electrode region.
Another known approach to widening the viewing angle is illustrated in FIG. 2A. In this approach each pixel electrode 13 is divided into two domains 13a and 13b, for example. The liquid crystal in the domain 13a adjacent the surface on the side of the transparent substrate 17 is made to have a large pretilt angle while the liquid crystal in the domain 13b adjacent the surface on the side of the transparent substrate 17 is made to have a small pretilt angle. On the other hand, the liquid crystal in the domain adjacent the surface on the side of the transparent substrate 16 opposing the domain 13a is made to have a small pretilt angle whereas the liquid crystal in the domain adjacent the surface on the side of the transparent substrate 16 opposing the domain 13b is made to have a large pretilt angle. The small pretilt angle may be provided by diagonal vapor deposition of a non-organic layer 31 such as SiO.sub.2 or by formation of polyimide film for a low pretilt angle. The large pretilt angle may be provided by subjecting an organic layer 32 such as polyimide resin for a high pretilt angle to a rubbing treatment. (see SID 92 DIGEST. pp. 798-801: Reference 1.)
Still another method of widening the viewing angle has been proposed as illustrated in FIGS. 2B and 2C. In this method, each pixel electrode 13 is divided into a plurality of, say two domains 13a and 13b having opposite rubbing directions. The domains on the side of the common electrode 12 opposing the domains 13a and 13b, respectively are made to have the same rubbing directions 35, 36 and perpendicular to the rubbing directions 33, 34, respectively of the domains 13a, 13b. The rubbing directions 33, 34 are imparted to the polyimide resin layer 37 for a high pretilt angle while the rubbing directions 35, 36 are imparted to the polyimide resin layer 38 for a low pretilt angle. (see JAPAN DISPLAY '92, pp. 591-594: Reference 2.)
FIG. 3 shows the up and down (negative and positive) viewing angle characteristics of a conventional normally white type TN LCD panel, having no special arrangements for widening the viewing angle, for the various activated states in which the designed normalized brightness of 0, 25, 50, 75 and 100% in the vertical direction are provided by varying the driving voltage applied to the panel with the maximum designed brightness at 100%. In the graph, the abscissa represents the viewing angles to the plain of an upright LCD panel, assuming that the horizontal direction be 0.degree. whereas the ordinate represents the relative values of brightness as measured. As seen in FIG. 3, as the designed normalized brightness lowers, the viewing angle which maximizes the relative brightness increases with respect to the horizontal direction (that is, the angle at which one looks down the LCD panel becomes larger). It is also noted that on the side of the negative (up) viewing angle, peaks ascribed to retardation appear in the respective curves, and then the viewing angles in the respective curves at which the minimal brightness is exhibited become larger in the negative direction with an increase in the designed normalized brightness. Due to such asymmetrical characteristics, gray scale reversal takes places at and beyond about .+-.30.degree.. It is seen, for instance, that the phenomenon of gray scale reversal in which the order of magnitude of the actually exhibited brightness is reversed occurs at about+40.degree. for the designed normalized brightness of 25, 50, 75 and 100%.
With the gray scale LCD panel as shown in FIGS. 1A and 1B, while gray scale reversal in its up and down viewing angle characteristics is less likely to occur as seen from FIG. 4, the problem of the viewing angle characteristics being asymmetrical between the up and down (negative and positive) directions, is not alleviated.
The up and down viewing angle characteristics of the domain-divided TN LCD panel as illustrated in FIG. 2A exhibits symmetry between the up and down directions as seen from FIG. 5. However, gray scale reversal arises at and beyond about .+-.40.degree.. It is also seen that for the designed normalized brightness of 0%, a relatively high brightness is provided at and beyond about .+-.40.degree. with the contrast lowered.
The domain-divided, oriented TN LCD panels as illustrated in FIGS. 2B and 2C also exhibit similar characteristics.